1. Field of the Invention
The present invention relates to a continuous melt-plating apparatus and, more particularly, to a continuous melt-plating apparatus suitable for adjusting the flatness of a gas wiping portion of a steel strip subjected to a continuous melt-plating method in which gas wiping is effected.
2. Description of the Prior Art
Conventional continuous melt-plating methods include: a type of method in which the steel strip is not subjected to acid cleaning or flux treatment, but is surface cleaned by performing oxidation and reduction before plating; and another type in which acid cleaning and flux treatment are performed before plating. An example of a method of the former type is disclosed in Japanese Patent Unexamined Publication No. 61-147900.
In a continuous melt-plating apparatus for carrying out such a continuous melt-plating method, an arrangement for maintaining the flatness of the steel strip in the gas wiping portion is disclosed in, for instance, Japanese Patent Publication No. 45-41085. In this arrangement, two guide rollers are provided between the gas wiping nozzle and a sink roller disposed below the nozzle, with one of the guide rollers being positioned lower than the other. The lower guide roller is adjusted in such a manner as to be offset from the mating guide roller, so that widthwise curving resulting from the rising of the steel strip can be corrected in order to maintain the flatness of the steel strip right at the portion opposing the gas wiping nozzle.
Gas wiping has been developed for use in plating a steel strip with a melt such as zinc, aluminum and nickel, and since it has various advantageous features, gas wiping is at present adopted in almost all the plating methods in this field. When gas wiping is to be effected with a view of blowing off and wiping off an excess of the plated melt layer, it is important to give consideration to the fact that the amount by which the melt can be blown and wiped off is greatly varied depending on the gap between the nozzle and the steel strip (i.e., the gap between the gas injection port and the steel strip). Certain experiments have shown that the resultant thickness .DELTA.t of a plating is expressed by the following formula:
.DELTA.t.varies.C.multidot..sqroot..delta.where .DELTA.t: thickness of plating
.delta.: gap between the tip of the nozzle and the steel strip
C: constant
Therefore, if the gas wiping portion of a steel strip has any irregularities occurring in the widthwise direction thereof, the irregularities cause corresponding variations in the gap between the steel strip and the nozzle and, hence, variations in the thickness of the plated layer. When the thickness of a plating is to be set, because the thickness of the thinnest portion has to be used as the reference from the viewpoint of assuring the performance of the plating, the thickness is inevitably set to a rather large value capable of compensating for those possible variations in the thickness of the plating. Thus, the thickness of a plated layer includes a margin corresponding to variations therein, which is termed a dead thickness. The thickness of the plating is also affected by the waving and curving of a portion of the steel strip which moves above the nozzle. In particular, when the steel strip has a relatively small thickness, curving occurs severely. Since a portion of the steel strip which has left the plating path cannot be held by, e.g., rollers until it is cool, the portion gradually curves in the widthwise direction after leaving the roller in the plating bath because of widthwise difference in thermal expansion resulting from changes in temperature generated during the plating. In order to compensate for the widthwise difference in expansion and also to center the steel strip, a crown is often provided for the sink roller within the plating bath. However, such a crown itself often causes the curving and waving of the steel strip. A high degree of curving amounts to about .+-.20 mm. Since the gap between the nozzle and the steel strip generally averages about 30 to 50 mm, there is a risk of large variations being caused in the thickness of the plated layer.
Curving leads to the following problem as well. When the plated melt layer and the nozzle opening are brought into mutual contact by attracting action between the tip of the nozzle and the curved steel strip, part of melt in the plated layer adheres to the nozzle opening, resulting in clogging or other disadvantages.
In order to overcome these problems, Japanese Patent Publication No. 45-41085 proposes a solution in which curving is corrected by means of the offset between or the overlap of the two guide rollers provided between the sink roller and the gas wiping nozzle. According to this proposal, however, since the curving of the steel strip is corrected solely by the overlap of straight rollers, the amount and the configurations provided by this correction are inevitably limited. Thus, the proposal has not been able to correct very large curving or complicated waving. Also, the degree of precision with which the gas wiping portion is kept flat has not been sufficient. Hitherto, because the plating speed (i.e., the speed at which the steel strip is passed) has been relatively low (i.e., approximately 50 to 100 m/min at most), the proportion in which the gap between the nozzle tip and the steel strip is varied has not been very large even if the gap is relatively large and the precision of the flatness is poor. It has therefore been possible to achieve thickness of platings which is uniform to a substantially satisfactory degree. In the case of the above-described prior art, although the correcting ability of the guide rollers is limited and, in addition, the difference in height between the guide rollers and the gas wiping nozzle is restricted to 300 mm or below, no problem has been encountered in practice.
Although gas wiping was at first used in the molten-zinc plating lines, as the application of gas wiping broadens during the passage of a long period into almost all the platings of Zn, Al, Ni, etc., an increasingly higher level of performance has been required. Currently, it is clearly seen that there are strong demands for, e.g., the achievement of plating thickness which is uniform to a higher degree with a view to saving resources and reducing the unit, and for the enhancement of the plating speed and, hence, the production efficiency. In order to enhance the plating speed (i.e., the steel strip passing speed) with the same thickness of the plating, it is necessary either to bring the tip of the gas wiping nozzle closer to the steel strip or to increase the gas discharge pressure. Since an increase in the gas pressure leads to an increase in the unit, and also leads to an increase in the noise generated in the vicinity of the plating bath and, hence, to deterioration in the working environment, the present situation is such that, on the contrary, the gas pressure is gradually lowered. For these reasons, in order to achieve thin platings at high speed, the gap between the tip of the nozzle and the steel strip must be much smaller than that conventionally provided. When the steel strip passing speed is increased, this causes an increase in the amount by which plating melt material (e.g., melt zinc) in the plating bath is attached to and thus raised by the steel strip. Therefore, it is necessary to increase the height of the gas wiping nozzle from the plating bath, and allow the excess of the plated layer to quickly drop off by its own weight, for the purpose of making it easy for blowing and wiping by gas wiping to achieve a thin thickness and for preventing the plating melt from scattering toward the nozzle and, hence, from causing clogging.
In the case where a sink roller is combined with a bearing portion disposed below the surface of the melt in the bath, plain bearings are in general used to form the bearing structure, as disclosed in Japanese Patent Unexamined Publication No. 54-18430. However, since the bearing surfaces are subjected to severe corrosion by the molten zinc, wear occurs in a short period. This has led to a loose fitting, with which the sink roller greatly vibrates in the transverse direction, resulting in great variations in the gap between the tip of the gas wiping nozzle and the steel strip and, hence, variations in the thickness of the plating. In order to avoid this risk, an arrangement is adopted in which the bearing portion is disposed above the upper surface of the bath melt.
In this way, in compliance with the demand for a drastic reduction in the gap between the tip of the nozzle and the steel strip, a higher degree of precision is achieved for the flatness of the gas wiping portion of the steel strip in the widthwise direction thereof.